"Tab Preview" is a new extension for Firefox 1.5 (and later) that shows a tab content (page) preview as you move your mouse pointer to a tab:
This comes very handy when you have lots of different tabs, especially on the same topic and similar titles.
This extension is quite configurable through "Tools -> Add-Ons -> (extension) Options":
Try it out, it is very fast and adjusts the size of its previews automatically depending on a browser window size.
I found it to be a very cool one – install it to experience its beauty :)